We describe quantum wires and point contacts fabricated in GaAs/AlxGa1−xAs heterostructures that are free of the disorder introduced by modulation doping and in which the electron density and the confining potential are separately adjustable by lithographically defined gates. We observe conductance plateaus quantized near even multiples of e2/h in 2 μm wires and up to 15 conductance steps in 5 μm wires at temperatures below 1 K. Near the conductance threshold the quantum point contact and the 2 μm wire both show additional structure below 2e2/h.

Zero length quantum wires (or point contacts) exhibit unexplained conductance structure close to 0.7 × 2e 2 /h in the absence of an applied magnetic field. We have studied the density-and temperature-dependent conductance of ultra-low-disorder GaAs/AlGaAs quantum wires with nominal lengths l=0 and 2µm, fabricated from structures free of the disorder associated with modulation doping. In a direct comparison we observe structure near 0.7 × 2e 2 /h for l=0 whereas the l = 2µm wires show structure evolving with increasing electron density to 0.5 × 2e 2 /h in zero magnetic field, the value expected for an ideal spin-split sub-band. Our results suggest the dominant mechanism through which electrons interact can be strongly affected by the length of the 1D region.73.61.-r, 73.23.Ad, 73.61.Ey III-V

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We report developing coplanar waveguide devices which can perform dielectric spectroscopy on biological samples within a microfluidic channel or well. Since coupling to the fluid sample is capacitive, no surface functionalization or chemical sample preparation are required. Data on cell suspensions and solutions of proteins and nucleic acids spanning the frequency range from 40 Hz to 26.5 GHz are presented. Low-frequency data are well explained using a simple dispersion model. At microwave frequencies, the devices yield reproducible and distinguishable spectral responses for hemoglobin solution and live E. coli.

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We report on cyclotron resonance detected by far-infrared photoconductivity of the two-dimensional electron gas formed in undoped, top-gated GaAs/Al 0.3 Ga 0.7 As heterostructures. The photoconductivity method demonstrated here is readily extendable to quantum wires. The top-gated device architecture avoids the disorder inherent in conventional modulation-doped devices and allows precise in situ tuning of carrier density over two orders of magnitude. We observe very sharp resonances (6 mT at 1.5 K) indicating a very high mobility, which is attributed to the low level of impurities. The variation of the linewidth at small filling factor is also consistent with a low concentration of impurities. These results suggest that the filling-factor-dependent oscillations observed in linewidth are not due to the screening of ionized impurities. Filling-factor-dependent oscillations in photoconductivity intensity are also observed, with maxima occurring at even filling factors.